There were four takes, starting at 8:30 p.m. If the shot was too
early, the lighting wouldn’t work; too late, the crowd would
thin. Crew was kept to a minimum, to draw as little
attention as possible. Keaton’s movements were accompanied by
only four people: Lubezki [cinematographer]; the focus puller;
the boom operator; and the digital imaging technician. Eight
production assistants worked on crowd control. Inarritu was close
by; for two of the four takes, he shot Keaton with his smartphone
for footage used in a subsequent scene in which Emma Stone
watches the incident on YouTube...

Because production couldn't afford to shut down
Times Square or fill it
with paid extras, real fans and onlookers became part of the
shot.

"We worried about security,"
Inarritu told Variety, adding that there was a fear someone would
stare at the camera or walk into the scene. "There was
no possibility to cut away if that happened" since it was one
long take, the director notes. "If any scene in the film failed,
I could not remove it or manipulate it. It had to be
perfect."

Here's what the shoot looked like in reality as a passerby
in Times Square:

In order to divert people's attention from the
camera, Variety reports Inarritu hired a group of street drummers
who danced and performed nearby. "All the tourists wanted to look
at these drummers. A half-naked man in Times Square? They’ve seen
that before," said the director.

"Birdman" production designer, Kevin
Thompson, explained further to Yahoo Movies:

"Having the drummers there really assisted in gathering and
holding the crowd, and then also holding space along one side of
them. It created an energy that helped, I think."

Now watch the shoot from the drummers' point of
view:

"Even though it does look like chaos, we did have to
control the crowd and extras," added Thompson. "For the most part
we just had a ton of film crew dressed as pedestrians that we’d
guide, and then all these extras taking pictures. It was very
complicated."

Another challenge that
production faced was all of the prominent brand advertising in the
background of the Times Square shoot.

The "Birdman" legal team had to get permission from each
brand to be used onscreen, reports Variety.

"Everybody showed up every morning frightened," Keaton told EW.
"The crew too. I think we were all thinking, I don't want to be
the guy who lets everybody down."

The cast, crew, and camera team had to be in sync at all times on
the very fast 30-day shoot.

Here's how EW describes what one mistake would cost them on
set:

"Anything—a misremembered line, an extra step taken, a
camera operator stumbling on a stair or veering off course or out
of focus—could blow a take, rendering the first several minutes
unusable even if they had been perfect."

"You had to be word-perfect, you had to be off script, and
you literally had to count your paces down to the number of steps
you needed to take before turning a corner," Keaton told
EW.

But with four Oscar wins, the challenging shoot was clearly
worth it.

The small film has been profitable, too. On a production budget
of just $18 million, "Birdman" has raked in
over $76 million worldwide since its late October
release.